The present invention generally relates to radiotelephone systems, and more particularly to a radiotelephone system having a base unit and a plurality of subordinate units for carrying out radiocommunication between the base unit and the subordinate units.
A conventional radiotelephone system usually performs carrier sensing to find a particular carrier frequency (channel) which is not in use at that instant among a number of given carrier frequencies (channels), and assigns the sensed free frequency to the base unit and to the subordinate unit (hereinafter referred to as a sub-unit) as a sending carrier frequency to be used for the communication, thus preventing radio interference with another radiotelephone system. In this manner, the conventional radiotelephone system enables a radiocommunication to be carried out between the base unit and the sub-unit without causing radio interference.
However, in the conventional system discussed above after the carrier sensing, the base unit and the subordinate units continuously make use of a fixed sending carrier frequency or a fixed receiving carrier frequency during radiocommunication. And the contents of telephone communication are transmitted and received via a carrier wave, and thus there is a problem in that using a common receiver allows any person to pick up information of the radiocommunication from the carrier wave. And, for those who operate such a radiotelephone system, it is a fear that the privacy during telephone communication is not always kept.
And a radiotelephone system having a base unit and a plurality of sub-units usually provides the capability of holding a so-called telephone conference among the sub-units or among the sub-units plus an external unit connected to a telephone line of the base unit. In order to hold such a telephone conference among, for example, first and second sub-units and the external unit, the base unit has to send a mixed speech or voice between the first and second sub-units to the external unit, send another mixed speech between the external unit and the first sub-unit to the second unit, and send still another mixed speech between the second sub-unit and the external unit to the first sub-unit. The first and second sub-units have to send each speech to the base unit. Therefore, four different channels are required for such a radiotelephone system to carry out the telephone conference. If one more sub-unit is installed on the radiotelephone system, two additional channels are required to allow such a sub-unit added to take part in the telephone conference. The larger the system becomes, the broader the bandwidth becomes. Because of this, the conventional radiotelephone system usually has to employ a broad bandwidth, which arises a problem in putting the system into practical use.